Absorption in packed bubble columns

The mass transfer characteristics of packed bubble columns were studied by employing various packings of different sizes and shapes in 10–38·5 cm i.d. columns. The theory of absorption accompanied by pseudo-mth order reaction was used to obtain the values of effective interfacial area. The values of liquid side mass transfer coefficient were obtained by using the theory of absorption accompanied by slow chemical reaction. The superficial gas velocity was varied from 5 to 25 cm/sec. The packed bubble columns showed a considerable improvement in the performance over empty bubble columns. A criterion for the scale-up of these columns has been suggested.     

Long-term prediction of nonlinear hydrodynamics in bubble columns by using artificial neural networks

An artificial neural network (ANN) model was proposed for the long-term prediction of nonlinear dynamics underlying holdup fluctuations in bubble columns with three different diameters of 200, 400 and 800 mm. Local holdup fluctuations were measured with an optical probe in the bubble columns. The superficial gas velocity was varied in the range of 33–90 mm/s. The time intervals between successive bubbles were extracted from the time series of holdup fluctuations to represent hydrodynamic behaviors in the system and used as training and validation data sets. The effect of data preprocessing as well as the numbers of nodes in input and hidden layers on the ANN training behavior was systematically investigated. The prediction capability of the ANN was evaluated in terms of time-averaged characteristics, power spectra and Lyapunov exponents. It was observed that: the ANN model, which was trained with experimental time series and gas velocity, can be used for the long-term prediction of dynamic characteristics in bubble columns by using random data as the initial input. The results indicate that the trained ANN models have the potential of modeling nonlinear hydrodynamic behaviors in bubble columns.     

Bubble size distribution in bubble columns

The bubble size distributions are measured for the air-water system as a function of air velocity at room temperature in two bubble columns. High speed cinephotography and fiber optic probe techniques are used to measure the bubble size. Our limited measurements suggest that the bubble size may be independent of gas velocity in the range 3.6 to 9.2 cm/s and may be dependent on column diameter with smaller bubbles for narrower columns. The bubble size appears to be smaller at the column wall than at distances away from the wall.     

Hydrodynamic and mass transfer characteristics of bubble and packed bubble columns with downcomer

The hydrodynamic and mass transfer characteristics of bubble and packed bubble columns with downcomer were investigated. The contactor consisted of two concentric columns of 0.11 and 0.2 m i.d., with the annulus acting as the downcomer. The packing used in this investigation was standard 16 mm stainless steel Pall rings. The superficial gas and liquid velocities, V_G and V_L, were varied from 0.01 to 0.09 and 1 × 10^?3 to 8.8 × 10^?3 m s^?1 respectively. Two flow patterns, namely the bubble and pulse flows were observed in the packed bubble column with downcomer, as shown by a flow map. The liquid circulation velocity in both the contactors was observed to be constant throughout the ranges of V_G and V_L covered in this work. The effect of liquid viscosity (0.8 to 9.5 mPa ? s) and surface tension (45 to 72 mN m^?1) on the flow pattern, liquid circulation, gas hold-up and pressure drop was investigated. The pressure drop characteristics across the two contactors have been compared with those across a bubble column. Values of the effective interfacial area, a, and the volumetric mass transfer coefficient, k_L a, were measured by using chemical methods. Values of a as high as 180 and 700 m^?1 and k_L a as high as 0.075 and 0.22 s^?1, in the bubble and packed bubble columns with downcomer, respectively, were obtained. The values of true liquid-side mass transfer coefficient, k_L, were found to be independent of V_G and were of the order of 5.5 × 10^?4 and 3.5 × 10^?4 m s^?1, respectively, in the two contactors.     

Bubble swarm behaviour and gas absorption in non-newtonian fluids in sparged columns

Mean relative gas hold up, slip velocity, bubble size distribution, and volumetric mass transfer coefficient of oxygen were measured in sparged columns of highly viscous non-Newtonian fluids (CMC solutions) as a function of the gas flow rate, and CMC concentration (fluid consistency index k, and flow behaviour index n).By comparison of the measured bubble swarm velocities with those calculated by relations for single bubbles the bubble swarm behaviour was investigated. It could be shown that small bubbles in swarm have higher rising velocities than single bubbles, expecially in highly viscous media. Large single bubbles rise with high velocity due to the change of their shape caused by the swarm of the smaller bubbles. No large bubbles with spherical cap shape could be observed. The volumetric mass transfer coefficient decreases rapidly with increasing CMC-concentration.A comparison of the volumetric mass transfer coefficients with those measured in mechanically agitated vessels indicates, that the performance of sparged columns is comparable with the one of agitated vessels. Because of their lower energy requirement sparged columns are more economical than mechanically agitated vessels. It is possible to improve the performance of sparged columns by the redispersion of large bubbles in a multistage equipment.     

Liquid mixing by large gas bubbles in bubble columns

Liquid mixing by large gas bubbles of spherical cup was investigated for co- and counter-current contact of air-water system with bubble columns of 5 and 10 cm dia. The results obtained are that for the column of 5 cm dia., the longitudinal dispersion coefficient ranges from 5 to 20 cm²/sec for superficial gas velocity from 0·07 to 8 cm/sec and that for the one of 10 cm in diameter it ranges from 9 to 45 cm²/sec for that from 0·035 to 8 cm/sec. Liquid mixing under the coexistence of large and small bubbles was also investigated and it was found that the gas holdup was fairly well explained by an equation derived on the assumption that the mixture of small bubbles and liquid behaves independently of large bubbles. The expansion model was applied to the experimental results on the longitudinal dispersion coefficient and it was observed that there should be the lower limit in the holdup of small bubbles where this model can be applied.     

鼓泡塔气液两相流内部流场的流体力学特性

采用计算流体力学CFD软件对鼓泡塔内部4种表观气速下内部流场的流体力学行为进行模拟。分析了鼓泡塔内部整体气含率及轴截面处（X=0）液相速度随时间的变化情况;并且对比了在不同表观气速下局部气含率和液相速度在不同高度处的径向分布情况。模拟结果表明,随着时间的增大,整体气含率增大速度比较快,到达稳定时间,整体气含率不再增大。在同一高度处局部气含率随着表观气速的增大而增大。H/D<3时,液相循环流动表现为单相循环流;当H/D=3时,表现为双循环流,流型较单相复杂。     

Experimental and numerical investigations of scale-up effects on the hydrodynamics of slurry bubble columns

Experiments and simulations were conducted for bubble columns with diameter of 0.2 m(180 mm i.d.), 0.5 m(476 mm i.d.) and 0.8 m(760 mm i.d.) at high superficial gas velocities(0.12–0.62 m·s-1) and high solid concentrations(0–30 vol%). Radial profiles of time-averaged gas holdup, axial liquid velocity, and turbulent kinetic energy were measured by using in-house developed conductivity probes and Pavlov tubes. Effects of column diameter, superficial gas velocity, and solid concentration were investigated in a wide range of operating conditions. Experimental results indicated that the average gas holdup remarkably increases with superficial gas velocity, and the radial profiles of investigated flow properties become steeper at high superficial gas velocities. The axial liquid velocities significantly increase with the growth of the column size, whereas the gas holdup was slightly affected. The presence of solid in bubble columns would inhibit the breakage of bubbles, which results in an increase in bubble rise velocity and a decrease in gas holdup, but time-averaged axial liquid velocities remain almost the same as that of the hollow column. Furthermore, a 2-D axisymmetric k–ε model was used to simulate heterogeneous bubbly flow using commercial code FLUENT 6.2. The lateral lift force and the turbulent diffusion force were introduced for the determination of gas holdup profiles and the effects of solid concentration were considered as the variation of average bubble diameter in the model. Results predicted by the CFD simulation showed good agreement with experimental data.     

LIQUID PHASE MIXING IN MECHANICALLY AGITATED VESSELS

Liquid phase mixing and power consumption have been studied in 0.3, 0.57, 1.0 and 1.5 m i.d. mechanically agitated contactors. Tap water was used as liquid phase. The impeller speed was varied in the range 2-13.33 r/s. Three types of impellers namely disc turbine (DT), pitched turbine downflow (PTD) and pitched turbine upflow (PTU) were employed. The impeller diameter to vessel diameter ratio was varied in the range of 0.25 to 0.58. The effect of impeller clearance from tank bottom was also studied. Mixing time was measured using the transient conductivity measurement.

The PTD impeller was found to be the most energy efficient for mixing in liquid phase alone. Further, PTD (T/3) was found to be most energy efficient as compared with other impeller diameters. The effect of clearance was found to be design dependent and it was found to be diameter dependent in the case of pitched turbines.

Flow patterns of different impellers have been studied by visual observations (using guide particles). These observations were supported by the measurements using Laser Doppler Velocimetry. A model has been developed for the prediction of mixing time. In the case of all the three impeller designs, a fairly good agreement was found between the predicted and experimental values of mixing time.     

Drag models coupled with CFD–PBM method for simulation in bubble columns

In this study, three-dimensional numerical simulation of gas–liquid flow in bubble columns was realized by using the computation fluid dynamics (CFD)–population balance model (PBM). The new drag model improves the stability-constrained multi-fluid (SCMF-C) model because of the consideration of the wake accelerating and the hindering effects for calculating the drag correction factor. The gas holdup, axial liquid velocity, and bubble size distribution (BSD) predicted by four drag models at 0.02 and 0.1 m/s were compared. The results revealed that the proposed drag model can provide excellent predictions for both bubbly and heterogeneous flows. Because the wake accelerating and the hindering effects were considered, reliable predictions were achieved for the gas holdup, and the problem of uniform gas holdup distribution was mitigated. Therefore, the SCMF-C model can be extended for nonuniform BSD. The gas holdup and liquid velocity increased, and the nonuniformity of radial results became pronounced at 0.1 m/s. The profiles of four drag models were similar at a low height, whereas the difference between the simulations of the four models became obvious with the variation of heights. The results of the four models were accurate, and the BSD was wide at 0.1 m/s. Subsequently, the feasibility of the four drag models was evaluated at 0.2 and 0.4 m/s. The results of the comparison revealed that the proposed drag model exhibited excellent feasibility at higher gas velocities and was powerful for the simulation of bubble columns.     

Mass transfer in bubble columns packed with motionless mixers

The cocurrent upward mode was employed to absorb pure oxygen into water in bubble columns packed with Koch (Sulzer) motionless mixers. The liquid-side volumetric mass transfer coefficient, K_La, in the packed bubble column was found to be always larger than that in the unpacked bubble column. In the range of liquid velocities from 6.7 cm/sec to 39.9 cm/sec, the value of K_La in the packed bubble column increased with the increasing liquid velocity while that in the unpacked bubble column was almost independent of the liquid velocity. The equation of the formK_La= mν_l^β? was successfully adopted to correlate the K_La data.     

Intensified operation of slurry bubble columns using structured gas injection

We investigate uniform gas injection using a needle sparger as a structuring methodology to reduce backmixing in slurry bubble columns. Using optical probes, we determined the gas fraction and the bubble behaviour in 2D and 3D slurry bubble columns with a uniform gas injection. Experimental results for air–water–glass beads (d_s = 108 µm, U_sg = 0–0.10 m/s) indicate that a strong reduction in the vortical structures has been achieved and the homogeneous flow regime can be extended beyond 30% gas fraction. Increasing the solids concentration decreases the gas fraction and widens the bubble velocity distribution. Furthermore, we show by modelling that the reduced backmixing leads to a major improvement of the conversion in case of Fischer–Tropsch synthesis.     

Liquid phase axial mixing in bubble columns operated at high pressures

Liquid phase axial mixing was measured in a 100 mm i.d. bubble column operated in the pressure range of 0.1-0.5 MPa. Water, ethanol and 1-butanol were used as the liquid phase and nitrogen as the gas phase. The temperature and superficial gas velocity were varied in the range of 298-323 K and 0.01-0.21 m/s, respectively. The axial dispersion coefficient increased with an increase in the gas density due to pressure. The temperature had surprisingly a small effect. A CFD model was developed for the prediction of flow pattern in terms of mean velocity and eddy diffusivity profiles. The model was further extended for the prediction of residence time distribution and hence the axial dispersion coefficient (D_L). The predictions of axial dispersion coefficient agree favorably with all the experimental data collected in this work as well as published in the literature. The model was extended for different gas-liquid systems. The predicted values of axial dispersion coefficient were found to agree very well with all the experimental data.     

Solid hold-up in bubble columns with suspended solid particles under continuous operation

Solid hold-up in bubble columns with suspended solid particles was studied experimentally under continuous slurry feed for the system, air–water–glass beads, in two columns of 55 mm and 95 mm in diameter. Experimental results agree well with the prediction made on the basis of the one-dimensional dispersion model.     

Mass-transfer-effective bubble coalescence frequency and specific interfacial area in a mechanically agitated gas-liquid contactor

This paper describes a new technique for concurrently evaluating the mass-transfer-effective bubble coalescence frequency and specific interfacial area of mechanically agitated gas-aqueous electrolyte solution dispersions by the chemical method. The technique is based on a model of mass-transfer-effective bubble interactions which permits computation of a mass transfer driving force correction factor. The latter is shown to be dependent on both the effective bubble coalescence frequency and the fractional depletion of the solute gas. The correction factor is applied to the experimentally-measurable exit gas composition in order to determine the relevant residence-time average driving force. Results are given for the effects of impeller rotational speed or mechanical agitation power, gas superficial velocity and total ionic strength on the mass-transfer-effective bubble coalescence frequency and specific interfacial area.     

Enhancement in gas holdup in bubble columns through use of vibrating internals

Including internals in bubble columns is known to enhance the gas holdup. In this paper, a method to achieve this objective substantially has been proposed via the use of vibrating helical spring internals. Experimental observations on effect of vibrating internals such as vibrating helical springs on gas holdup in bubble columns are presented. Effects of superficial gas velocity, H/D ratio (height of the static liquid to column diameter ratio), volume fraction of helical springs, and thickness of the helical spring wires on hydrodynamics parameters are studied. Increase in gas holdup up to 135% is observed by using vibrating helical spring internals in bubble columns compared to bubble columns without internals. This method offers a simple, cost‐effective, and easy way to enhance gas holdup even at high gas fluxes. It has been reported that this enhancement stems from the fact that the vibrating springs breakup the gas into fine bubbles, which effectively reduces their rise velocity and enhances their average residence time in the liquid column.     

Flow regimes and mass transfer in counter-current bubble columns

Counter current bubble columns have the feature that specific gas-liquid interfacial area and gas holdup are larger than those for standard and cocurrent bubble columns. In this study, three different flow regimes, churn-turbulent flow, bubble flow and bubble down-flow, have been observed in a counter-current bubble column and correlations of gas holdup and volumetric liquid-phase mass transfer coefficient have been proposed as functions of operating variables such as the superficial velocities of gas and liquid, the gas-liquid slip velocity and the liquid properties.     

EFFECT OF IMPELLER DESIGN ON LIQUID PHASE MIXING IN MECHANICALLY AGITATED REACTORS

Liquid phase mixing time (θ_mix) was measured in mechanically agitated contactors of internal diameter 0.57 m, 1.0 m and 1.5 m. Tap water was used as the liquid phase. The impeller speed was varied in the range of 0.4-9.0 r/s. Three types of impellers, namely disc turbine (DT), pitched blade downflow turbine (PTD) and pitched-blade upflow turbine (PTU) were employed. The ratio of impeller diameter to vessel diameter (D/T) and the ratio of impeller blade width to impeller diameter (W/D) were varied over a wide range. The effects of impeller clearance from the tank bottom (C), the blade angle (φ), the number of blades (n_b), the blade thickness (k) and the total liquid height (H/T) were studied in detail. Mixing time was measured using the conductivity method.

Mixing time was found to have a strong dependance on the flow pattern generated by the impeller. Mixing time was found to decrease by decreasing the impeller clearance in the case of DT and PTU. However in the case of PTD it increases with a decrease in the impeller clearance. Similar trend of the effect of impeller clearance on θ_mix, was observed for all the other PTD impellers with different diameter, number of blades and blade angle (except 60^° and 90^°). All the impeller designs were compared on the basis of power consumption and on this basis optimum design recommendations have been made. For PTD impellers, a correlation has been developed for the dimensionless mixing time.     

Bubble swarm characteristics in bubble columns

New correlations have been proposed for estimation of gas phase holdup, characteristic velocity, interfacial area for mass transfer and mean bubble size of bubble swarms under dispersed and fluidized operation of bubble columns employing single — and multi-orifice distributors. The analysis of results include available literature data of other investigators.     

Bubble swarm characteristics in bubble columns

New correlations have been proposed for estimation of gas phase holdup, characteristic velocity, interfacial area for mass transfer and mean bubble size of bubble swarms under dispersed and fluidized operation of bubble columns employing single — and multi-orifice distributors. The analysis of results include available literature data of other investigators.